Conventionally, a radio for microwave digital communication in STM-N (synchronous transport module-level N) transmission uses an MSP (multiplex section protection) system as a system for support of duplex of STM-N interface. The MSP system is described in, for example, the ITU-T Recommendation G.782 or G.783, etc.
The wireless communication system for performing wireless communication between the above-mentioned radios is explained below by referring to the (1+1) configuration which is the smallest configuration of the (N+1) configuration as the configuration of each radio. FIG. 1 shows the configuration of the conventional wireless communication system.
As shown in FIG. 1, the conventional wireless communication system is configured by radios 30a and 30b and MUX devices 101 and 102. The radio 30a is configured by an interface circuit 21a, a current transmitter/receiver 22a, a standby transmitter/receiver 23a, a circulator 24a, and an antenna 25a. The radio 30b is configured by an interface circuit 21b, a current transmitter/receiver 22b, a standby transmitter/receiver 23b, a circulator 24b, and an antenna 25b. 
The MUX devices 101 and 102 are connected to the respective node devices not shown in the attached drawings, and each of the MUX devices 101 and 102 multiplexes an input signal from a node device connected to it, branches the multiplexed signal (STM-N signal), and then transmits the two same branched STM-N signals to optical transmission lines 210 and 220 (250 and 260).
The two STM-N signals output from the MUX device 101 is input to the interface circuit 21a of the radio 30a through the optical transmission lines 210 and 220. The interface circuit 21a selects one of the two input STM-N signals, branches the selected signal into two signals for transmission through the current radio circuit and the standby radio circuit between the radios 30a and 30b, and then outputs the branched signals to the current transmitter/receiver 22a and the standby transmitter/receiver 23a. 
Each of the current transmitter/receiver 22a and the standby transmitter/receiver 23a modulates an input signal, converts the modulated signal to a radio frequency of an RF band, and then transmits the conversion result to the radio 30b which is an opposite station through the circulator 24a and the antenna 25a. The signal (the signal from the current transmitter/receiver 22a and the signal from the standby transmitter/receiver 23a) received through the antenna 25b of the radio 30b is input to the current transmitter/receiver 22b and the standby transmitter/receiver 23b through the circulator 24b. 
Each of the current transmitter/receiver 22b and the standby transmitter/receiver 23b converts an RF received signal to a signal of an intermediate frequency band, demodulates it, and outputs a base band digital signal which is a demodulation signal to the interface circuit 21b. The interface circuit 21b selects one of the two input base band digital signals from the current transmitter/receiver 22b and the standby transmitter/receiver 23b, branches the selected signals into two signals, and then outputs the branched signals to the MUX device 102 through the optical transmission lines 270 and 280.
The frequency distribution at the radio frequencies of the current radio circuit and the standby radio circuit between the radios 30a and 30b is the interleave distribution as shown in FIG. 2B. That is, the current transmitter/receivers 22a and 22b use the frequency F0 shown in FIG. 2B, and the standby transmitter/receivers 23a and 23b use the frequency F2 shown in FIG. 2B.
FIG. 3 shows the configuration of the interface circuits 21a and 21b shown in FIG. 1, and the components also shown in FIG. 1 are assigned the same reference numerals. As shown in FIG. 3, each of the interface circuits 21a and 21b is configured by STM-N input interface circuits 31 and 32, a selection circuit 33, a control circuit 34, a branch circuit 35, a selection circuit 37, a branch circuit 38, STM-N output interface circuits 39 and 40, and a CLK providing circuit 36. The CLK providing circuit 36 provides a generated clock to the STM-N input interface circuits 31 and 32 and the STM-N output interface circuits 39 and 40.
The two STM-N signals transmitted from the MUX device 101 to the optical transmission lines 210 and 220 are input to the STM-N input interface circuits 31 and 32 of the interface circuit 21a. Each of the STM-N input interface circuits 31 and 32 performs signal processing of MSOH (multiplex section overhead), which is an overhead signal of an input STM-N signal, and signal processing of transferring the input STM-N signal from the CLK providing circuit 36 to a clock to be provided. Each of the STM-N input interface circuits 31 and 32 monitors the quality of the input STM-N signal, and outputs the monitor result to the control circuit 34.
The control circuit 34 controls the selection circuit 33 to select a signal having better signal quality from between the two STM-N signals based on the monitor result from the STM-N input interface circuits 31 and 32. The selection circuit 33 selects a better signal from between the two signals from the STM-N input interface circuits 31 and 32, and outputs the selected signal. The branch circuit 35 branches the signal from the selection circuit 33 into two signals, and outputs them to the current transmitter/receiver 22a and the standby transmitter/receiver 23a. 
Meanwhile, the two signals output from the current transmitter/receiver 22a and the standby transmitter/receiver 23a are input to the selection circuit 37 of the interface circuit 21a. The selection circuit 37 selects a signal from the current transmitter/receiver 22a from between the two input signals, and outputs the selected signal. The branch circuit 38 branches the signal from the selection circuit 37 into two signals, and outputs the branched signals from the selection circuit 37 to the STM-N output interface circuits 39 and 40. When a failure occurs in the current system, the selection circuit 37 selects a signal from the standby transmitter/receiver 23a and outputs it.
The STM-N output interface circuits 39 and 40 of the interface circuit 21a convert the input signal from the branch circuit 38 to an STM-N signal, and transmit it to the MUX device 101 through the optical transmission lines 230 and 240. The operation of the interface circuit 21b is similar to the operation of the interface circuit 21a. 
[Patent Document 1]
Japanese Patent Laid-Open No. 2001-86051 (page 3, FIG. 1)